Base conversion: An Excel
spreadsheet with which you can experiment to see what numbers look
like when expressed in different bases. (I have set up this
spreadsheet so that you can only enter numbers, not change the
formulas that do the work. But if your version of Excel doesn't match
mine you might have to go to tools->unprotect to make this
spreadsheet work.)

Flip coins: An
Excel spreadsheet that simulates flipping ten coins at a time,
collecting and displaying statistics.